Traditional refrigeration systems include a compressor, a condenser, an expansion valve, and an evaporator, all interconnected for establishing series fluid communication therebetween. Cooling is accomplished through evaporation of a liquid refrigerant under reduced temperature and pressure. Initially, vapor refrigerant is drawn into the compressor for compression therein. Compression of the vapor refrigerant results in a higher temperature and pressure thereof. From the compressor, the vapor refrigerant flows into the condenser. The condenser acts as a heat exchanger and is in heat exchange relationship with ambient. Heat is transferred from the vapor refrigerant to ambient, whereby the temperature is lowered. In this manner, a state change occurs, whereby the vapor refrigerant condenses to a liquid.
The liquid refrigerant exits an outlet of the condenser and flows into the expansion valve. As the liquid refrigerant flows through the expansion valve, its pressure is reduced prior to entering the evaporator. The evaporator acts as a heat exchanger, similar to the condenser, and is in heat exchange relationship with a cooled area (e.g., an interior of a refrigeration case). Heat is transferred from the cooled area to the liquid refrigerant, thereby increasing the temperature of the liquid refrigerant and resulting in boiling thereof. In this manner, a state change occurs, whereby the liquid refrigerant becomes a vapor. The vapor refrigerant then flows from the evaporators, back to the compressor.
The cooling capacity of the refrigeration system is generally achieved by varying the capacity of the compressor. One method of achieving capacity variation is continuously switching the compressor between on- and off-cycles using a pulse-width modulated signal. In this manner, a desired percent duty cycle for the compressor can be achieved. During the off-cycles, liquid refrigerant experiences “freewheel” flow, whereby the liquid refrigerant migrates into the evaporator. As the refrigerant migrates into the evaporator during the off-cycle, it is boiled therein, and becomes a vapor. This is detrimental to the performance of the refrigeration system in two ways: a significant reduction in the on-cycle evaporator temperature, and a decrease in flow recovery once switched back to the on-cycle.
Further, significant losses occur with traditional refrigeration systems when recently compressed vapor reverse migrates through the compressor, back toward the evaporator, during the off-cycle. These losses are compounded by reverse migration of liquid refrigerant back into the condenser during the off-cycle.
Therefore, it is desirable in the industry to provide a refrigeration system and flow control strategy for alleviating the deficiencies associated with traditional refrigeration systems. In particular, the refrigeration system should prohibit migration of liquid refrigerant into the evaporator during the off-cycle, prohibit reverse migration of vapor refrigerant through the compressor during the off-cycle, and prohibit reverse migration of liquid refrigerant through the condenser during the off-cycle.